1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a surface acoustic wave resonator, a surface acoustic wave filter, a duplexer, or other such devices, and more particularly to a surface acoustic wave device using a Shear Horizontal (SH) type wave.
2. Description of the Related Art
Surface acoustic wave devices have been widely used for band-pass filters in mobile radio communication apparatuses and other types of communication devices. Various types of surface acoustic wave devices are commercially produced for various applications. Among such surface acoustic wave devices, a SH type surface acoustic wave device including a quartz substrate and an interdigital transducer (IDT) made of Ta, W or Au is disclosed in, for example, EP0860943A2.
In such a surface acoustic wave device, a metallic film is formed on a piezoelectric substrate that is made of quartz, or other suitable material, by evaporating or sputtering a metal such as Ta, W, and Au, and then electrode fingers defining the IDT of the surface acoustic wave device are generally formed by producing patterns of the metallic film by a method such as photo-etching. Resonance frequency of the surface acoustic wave device is mostly determined by the space between electrode fingers defining the IDT, the film thickness, and the width of the electrode fingers.
When the surface acoustic wave device is manufactured by the above-mentioned processes, a problem occurs in that the width of the electrode finger and the film thickness are different or varied in every wafer due to the lack of accuracy in controlling process parameters. These variations cause variations in frequency of the surface acoustic waves generated in the surface acoustic wave devices .
The inventors of the invention described and claimed in the present application discovered that, when the IDT is formed of a metal having a large density such as Ta, W, and Au, it has a serious problem of large variations in frequency due to variations in the width of the electrode fingers and the film thickness. More specifically, when the IDT is formed of Al which is very common to general surface acoustic wave devices, frequency deviations on a wafer are generally small. On the other hand, when the IDT is formed of a metal having a large density such as Ta, W, and Au, the frequency deviations are so great that the deviations on a wafer cannot be adjusted after formation of the IDT. This is because the frequency and the SAW velocity depend on the density of material for the IDT. Thus, the frequency deviations become very large even if the variations in thickness of the IDT are the same as the variations in the case of an IDT made of Al.
In order to inhibit the frequency deviations, it may be proposed that the thickness of a film made of Al having a small mass load is increased to such a degree that an SH wave can be generated. However, using the state of the art thin film forming techniques, there arises the problem that the deviations in the electrode finger width direction occurs, due to the distortion or the like of a resist film which is caused by the limit of the thickness of a resist film. For this reason, when the IDT is made of Al, it is difficult to increase the film thickness of the IDT to such a degree that an SH wave can be excited. Even if it is possible to form an IDT with a thickness that is sufficient to excite an SH wave, an electromechanical coupling coefficient obtained with such an IDT is too small to be used practically.
Further, it is generally estimated that the film thickness H/xcex (electrode thickness/wavelength of an SH wave to be excited) at which the electrode fingers of the IDT can be formed with high accuracy is within 5%. Regarding the attenuation constant (propagation loss) of a surface acoustic wave device of which the IDT is made of Al and has the above-mentioned film thickness, there arises the problem that the attenuation constant does not become zero.
In order to overcome the forgoing problems, preferred embodiments of the present invention provide a surface acoustic wave device having a very small change of the SAW velocity based on the film thickness, and the attenuation constant is nearly equal to zero measured just after the electrodes have been formed on the piezoelectric substrate.
According to one preferred embodiment of the present invention, a surface acoustic wave device includes a quartz substrate having Euler angles (0xc2x0, xcex8,90xc2x0xc2x15xc2x0) in which 114xc2x0xe2x89xa6xcex8xe2x89xa6140xc2x0, and an IDT for exciting an SH wave disposed on the quartz substrate and made of an electrode material containing as a major component at least one of Ag, Mo, Cu, Ni, Cr, and Zn and having a density of at least about 7 g/cm3, wherein the film thickness of the IDT is such that the attenuation constant is approximately zero.
When Ag is used as a major component for the electrode material, it is preferable that the normalized film thickness H/xcex of the IDT is about 0.021 to about 0.050.
When Mo is used as a major component for the electrode material, it is preferable that the normalized film thickness H/xcex of the IDT is about 0.016 to about 0.050.
When Cu is used as a major component for the electrode material, it is preferable that the normalized film thickness H/xcex of the IDT is about 0.027 to about 0.050.
When Ni is used as a major component for the electrode material, it is preferable that the normalized film thickness H/xcex of the IDT is about 0.019 to about 0.050.
When Cr is used as a major component for the electrode material, it is preferable that the normalized film thickness H/xcex of the IDT is about 0.014 to about 0.050.
When Zn is used as a major component for the electrode material, it is preferable that the normalized film thickness H/xcex of the IDT is about 0.028 to about 0.050.
According to a preferred embodiment of the present invention, the IDT is formed on a quartz substrate having Euler angles (0xc2x0, 114xc2x0xe2x89xa6xcex8xe2x89xa6140xc2x0, 90xc2x0xc2x15xc2x0) using an electrode material having a density of at least about 7 g/cm3. Accordingly, the resulting surface acoustic wave device has a very small change of the SAW velocity based on the film thickness, and the deviations of frequency are prevented.
Moreover, a surface acoustic wave device having an attenuation constant of about zero measured just after the electrodes have been formed on a piezoelectric substrate can be obtained by setting the normalized film thickness of IDT to a desired amount, the IDT being made of an electrode material containing as a major component at least one of Ag, Mo, Cu, Ni, Cr, and Zn and having a density of about 7 g/cm3.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.